An Analysis of Visibility and Anatomic Variations of Mandibular Canal in Digital Panoramic Radiographs of Dentulous and Edentulous Patients in Northern Iran Populations.

STATEMENT OF THE PROBLEM
Insufficient information about the anatomical positions and structure of mandibular canal provokes unwanted damage to this important structure of mandible.


PURPOSE
The aim of this study was to determine the visibility and anatomical variations of mandibular canal in digital panoramic radiographs of dentulous and edentulous patients in a sample of Iranian population.


MATERIALS AND METHOD
In this retrospective-analytical research, 249 digital panoramic radiographs in dentulous group and 126 in edentulous group were studied by an expert oral and maxillofacial radiologist. In both groups, the visibility of canal borders in anterior, middle, and posterior areas were examined. In dentulous group, the distance between the canal and apex of the first and second molars were measured. Canal-to-alveolar crest distance and lower mandibular border was measured in three different points for both groups. Finally, the upper-lower positions of canals were determined.


RESULTS
In both groups, most visibility occurred in 1/3 of posterior and the least visibility was detected in 1/3 of anterior, with the intermediate being the most visible part (Type 2). There was no significant difference between the left and right sides in all cases. In dentulous group, no correlation was found between the visibility, age, and gender (p> 0.05); however, canal position was related to gender (p= 0.03 and p= 0.04 in right and left sides, respectively). High position was more frequent in females and intermediate position was more common in males. In edentulous group, no correlation was found between age, gender, and canal position (p> 0.05).


CONCLUSION
The most visibility of mandibular canal was in its third posterior and the least was in its third anterior part. Although the middle position of canal was more frequently visible than the high position in this study, it does not refute the possibility of damaging the mandibular canal in critical surgeries.


Introduction
Radiographic diagnosis of a disease requires a precise knowledge of anatomic landmarks and natural structures. This kind of diagnosis cannot happen without considering the variations and alterations of natural anatomical structures. [1] The radiographic images and width of the mandibular canal show some variations among the patients. Sometimes the borders are only seen partially or not at all. [2] In maxillofacial surgeries, mandibular canal is considered a reference anatomic structure. Extracting the mandibular third molar, implant surgeries, orthognathic surgeries, and fixing the jaw fractures are cases with the high risk of damage to the mandibular canal and inferior alveolar nerve. [3] Proximity of the first and second molar root to the mandibular canal can cause injury to inferior alveolar nerve while extracting these teeth. [4] Inferior alveolar nerve may get traumatized in its intraosseous pathway. The most common place of injury is the third molar of lower jaw. Extracting the impacted third molar may result in nerve crushing or damage. [5] One side effect is dysesthesia, which includes paresthesia and anesthesia. This damage is related to deep impaction of tooth and proximity of roots to inferior alveolar nerve. [4,6] Inferior alveolar nerve can get damaged during endodontic or even orthodontic treatments. Over instrumentation or overfilling in mandibular premolar or molar teeth during endodontic treatment can cause nerve damage. Orthodontic movements of posterior mandibular teeth can impose pressure on inferior alveolar canal and even paresthesia. [7] Histologic studies have shown that the path of inferior alveolar nerve is typically in form of a main trunk (with sub-branches) toward the teeth apex in mandible.
But there are some smaller parts of inferior alveolar nerve that are almost parallel to the main branch. In some cases, they are so eminent that are considered as second mandibular canal. Two-branch mandibular canal (bifid) can be seen on panoramic radiographs and cone beam computed tomography (CBCT) images. Patients with bifid canals are at risk of inadequate anesthesia or some problems with jaw surgeries including dental implants. [2] Langlais et al. [8] divided the two-branch mandibular canal into four groups, regarding their anatomical position. Accordingly, in type 1, two-branch canal stretches toward the third molar or its surrounding either one-sided or two-sided. In type 2, the twobranches of canal are rejoined in ramus either one-sided or two-sided. Type 3 is the combination of type 1 and 2.
In type 4, two canals originate from separate mandibular foramen and join to form a large canal. Nortje et al. [9] divided the mandibular canal in three groups based on its superior-inferior position. In superior position, a single canal is in contact with the apex of the first and se-cond permanent molars or at its 2-mm distance. In case of their loss, the canal position should be considered in contact with the approximate position of the root apex of the first and second molars (based on the neighboring tooth). Inferior position includes a single canal in contact with or at a 2-mm distance from the inferior border of mandibular cortical plate. Middle position is between the superior and inferior positions. Considering the great usage of panoramic radiography in dentistry, interpatient variability of mandibular canal, and lack of any previous similar study, the present study was designed to assess the visibility and anatomical variations of mandibular canal in dentulous and edentulous patients and to compare these variations with respect to the side, age, and gender in Guilan Dentistry Faculty, Rasht, Iran.

Materials and Method
In this retrospective descriptive-analytical study, 375 panoramic radiographs which were comprised of 249 dentulous (124 male and 125 female) and 126 edentu- Helsinki and Tokyo for humans. We also obtained written consent from the participating patients to use their panoramic images for this study. All radiographs were taken by using a digital panoramic imaging system (Soredex Cranex-D TM ; Finland). They were analyzed on a 14" HD LED (1366×768) by using Scanora software The mandibular canal was evaluated in all radiographic images with the same digital processing with no alteration to the primary processing. Calibration was do-

Results
In dentulous group, the mean age was 26.  Table 2).
The most distant point to crest and inferior border of the mandible were point A and point C´, respectively.
The longest distance between the canal and apex of the first and second molars belonged to mesial root of   Independent t test also revealed that the measured distances in this group were not related to age (p> 0.05).
But on both sides, the canal distance to alveolar crest in points B and C, canal distance to mandibular border in A´, and canal distance to apex of the first and second molar were significantly longer in men (p< 0.05, independent t-test).
In edentulous group, the mean age was 58.80± 10.46 with the youngest being 30 and the oldest 90.
Similar to the previous group, for both genders and on both sides of the jaw, the most visibility of canal was in one-third of posterior and least was in one-third of anterior. In all the three parts of the canal, inferior border was more visible than the superior ( Table 4).
The most common position of the canal was intermediate (Type 2) and then high position (Type 1), with no canal being in low position (Table 5).
Based on the results of chi-square test, there was   (Table 6).
However, chi-square test showed a correlation between canal visibility and gender on both sides, that is, the superior border visibility in all the three parts of canal and inferior border visibility in one-third of anterior and one-third of middle was more in men than women (p< 0.05). By using independent t test and chisquare analysis, no significant correlation was found between age and gender with the canal position and measured distances on both sides (p> 0.05).

Discussion
In recent years, different studies have focused on normal landmarks and their natural structures for better identification of pathological lesions and diagnosis, and subsequently a better treatment plan. In this regard, neurovascular bundle in lower jaw is considered to be one of the most important and concerned landmarks. [11][12] It is necessary to have adequate and proper information about the variations in mandibular canal path and its topography. [13] Panoramic view is one of the most common radiographies in dentistry used by many dentists as a routine diagnostic imaging method in jaw problems and traumas. [14] In the present study, most visibility of inferior and superior borders in both groups, for both genders and on both sides, belonged to onethird of the posterior and the least visibility was in the one-third of anterior. These results may be due to changes in the pathway of mandibular canal to the buccal side before opening in mental foramen [15] which causes the anterior part being less clearly visible. This is in accordance with Angelopoulos et al.'s study [15] which compared the mandibular canal visibilities in Moreover, in the current study, in both groups and both genders, the visibility of inferior border was higher than the superior border; this is similar to Pria et al.'s [16] findings.
In the present study, in dentulous group, 15% of the canals on the right side and 10% of the canals on the left side were in high position (type 1). In dentulous group, in high position of the canal and in both sides, women had a relative superiority to men. This matter should always be remembered when observing the panoramic radiographs of women due to the high possibility of damaging to the canal while performing surgical, endodontic, orthodontic, and dental implant procedures.
In groups, genders, and sides, the most common posi- This study showed that in dentulous group, for both men and women and on both sides, the distance between the superior border of canal and alveolar crest was the longest between the second premolar and first molar. This is comparable to a research reported by Liu et al. [17] In their study, the longest distance between The present study found the shortest distance between inferior border of canal and inferior border of mandible to be between the first and second molars.
Again, this is comparable to Liue et al.'s findings. [17] In their study, the least distance between the canal and inferior border of mandible was in the first molar area, but quantitatively, it was a little more that the measure of our study (7.56±1.62 mm).
In dentulous group of our study, the distance be-  (Figures 4a and b).
Kumar et al. [22] evaluated the morphological variations of the mandibular canal in panoramic radiographs. In their study, the most common variations were bifid mandibular canal (4.3%) and double mandibular canal (4%) respectively. In comparison with the present study, the prevalence of bifid mandibular canal was higher. The authors elucidated that the disparity and overestimation of their results from the previous studies could be due to differences in the study design and ima- The findings of such studies may impact our findings.

Conclusion
In both dentulous and edentulous groups, the most visi-